Intermittent Spark in 1979 Rabbit

Hello all, I have a 79 Rabbit that suddenly developed an intermittent spark problem a few days ago. I am currently stranded in a nearby parking lot, but I've been able to do a fair amount of testing. After smelling gas, I figured I wasn't getting spark, so I hooked up a timing gun to the wires and found that I was only occasionally getting spark. I hooked it up to the wire coming out of the ignition coil, and discovered that it was/appears to be the source of the problem. I have two working rabbits, so I was able to switch out coils and wires, caps and buttons, but nothing changed. When I hook the gun up to the coil wire on my working rabbit, the light flashes quickly and regularly as it should. But on the non-working rabbit, the light will come on and off seemingly at random. Sometimes not at all. What else should I check/replace? Thanks!!!! Fred

Reply to
Fred Mann
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Have you replaced the points and condensor?

Reply to
Jim Behning

Oh no, I have points? Is this easy enough to do in the parking lot?

Reply to
Fred Mann

If you actually have points, the usual failure mode would be that the rubbing block that presses against the distributor shaft has worn, closing the gap. Loosen the points screw slightly, and pry so that the points are open about the thickness of a matchbook cover at max (when rubbing block is on peak of rotor cam). Tighten down the points, drive home, and replace properly later.

Reply to
Tom's VR6

I have had condensors go bad. It has been so long since I had points though. I could change points and condesor anywhere I had light, a magnet, a screwdriver and hopefully a clean smoth surface under the car. Make sure the plug is installed in the transmission where you time things. You hate to loose small parts in that hole. I remember a Pinto sitting in the parking lot at work for weeks. The lady said it would not start. I did what Tom mentioned. I looked at the points and they were not opening enough. I adjusted them a bit and off she went after an easy start up.

If I recall properly I upgraded my 1980 Rabbit distributor from points to electronic ignition. I had to change out the 12v power lead to the coil because the point sytem has a resistor wire in it. I had to get the coil, distributor, and ignition module from a 1981 maybe.

Reply to
Jim Behning

Well ... I changed the points and condenser (.019 gap) and even bypassed the ignition switch (hotwired it, basically). Still, intermittent to no spark. I am using a "used" wire from the coil to the distributor, but both wires work in my other rabbit and not in this one, so I'm assuming it's not the problem. Is there anything else to replace? Is there anything else that can effect spark? Thanks yet again!!! Fred

Reply to
Fred Mann

Does your dwell meter read anything when cranking? Does your voltmeter read anyting on the positive to the coil? Cranking should put as many volts to the positive as the battery has. There should be a jumper wire from the starter to the coil. Running volts should be less than 12 volts as the wire from inside the car is a resistor wire.

I had a coil wire go bad and leave me stranded. That would be the high tension wire from the coil to the distributor.I have had a coil go bad also. You really need to start with your multimeter to check primary voltage. Then a dwell meter. Then you should turn the crankshaft by hand with the distributor cap removed to make sure the distributor is turning.

Reply to
Jim Behning

Because you've tried everything else , meter the wire from/to the points/coil maybe you have a high resistance fault with the wire. You probably have a ballast resistor in series with the run postion of the ing. switch powering the coil but during cranking that's not in the circuit. Also - there is feed wire to the fuel pump relay , off of the wire from the pionts to the coil , maybe the relay is pulling down the voltage. You could just pull that relay out and test for good spark while cranking the starter. Pure guesses , as you've seemed to have covered everthing else.

Reply to
samstone

Thanks again, guys! I don't have a really good multi-meter. I do have a cheap one that would test voltages, but not dwell. Should I buy a better one? It seems like it would be quicker and cheaper just to replace the remaining parts/wires. Plus, I don't really know how to use a meter in these applications (i.e. where to hook up the leads and what to look for). I'll try pulling the relay if I can figure out which one it is -- I guess I'll just pull them individually until I hear the pump stop. Another thing I remembered is that I switched the starters from one rabbit to another. The 79 rabbit (the one I'm having trouble with) used to have the better starter, but I put it in my 83 GTI rabbit (the other starter was/is beginning to fail). BUT when I did so, there was a stray wire bound to the ground wire which didn't have a home. I just left it hanging, but the starter still worked okay. It worked fine like this for a while, but maybe this is now causing a problem? By the way, the distributor is definitely turning. Finally, is there any way to bypass any of the remaining wiring? At least for diagnostic purposes? Fred

Reply to
Fred Mann

The stray wire did not attach to a ground. It attached to the positve of the starter. That big wire going in to the starter is not a ground, it is 12v+. Ground is achieved like most every other ground on the car by the fact that the starter is made of metal and the engine is made of metal. There is a big ground strap from the battery to the frame under the battery. There is a big ground strap from that same attachment point to the transmission. There is a ground strap from the alternator to the engine block. There probably is a small ground strap or wire that attaches to the water housing adapter at the end of the head. As I recall there are no other grounds of signficance under the hood. Well there is a ground strap from the hood to the frame (figure of speach) to help reduce some igintion noise to the radio but that is not critical to engine running.That big wire from the solenoid would be the switched positve from the solonoid that feeds 12 volts to the starter. When you turn the ignition to start the solenoid is energized and makes a connection to the 12v+ of the starter. This loose wire is to be attached to the 12v+that feeds the starter motor which goes back to the coil providing as much voltage as the battery has to the coil. Why you ask? Remember I said you have a resistor wire that feed the primary voltage to the coil? Well that resistor wire give the coil maybe 9 volts when running based on the running battery voltage of

13.5 volts. When you start the car the battery voltage may drop down to say 11 volts. That resistor wire is still resisting so it may drop that voltage to the primary of the coil to say 7 volts. The coil is not going to make any good spark with such a low primary voltage. The jumper wire from the starter gives the as much voltage as the battery has which as I mentioned might be 11 volts when cranking. This enough for the coil to make a nice strong spark on the secondary.

Note that these voltages are theoretical. I do not know for sure how much the resistor wire drops the voltage. You have the car and the meter. Measure the battery voltage with the meter. Turn the ignition on. Measure the voltage on the primary of the coil. Attach the ground wire of your multimeter to a ground on the car someplace.

While you are there at the coil do this. Turn of the ignition. Set your multimeter to resistance. Measure the resistance from primary to chassis ground. Measure primary to

If you did not attach the wire from the starter to the coil did you at least put electrical tape on the starter end so that you would not overload the resistor wire with a short?

A dwell meter is not a voltage meter. Anyone who works on a car that has points and changes the points themself should own a dwell meter. A dwell meter is practically an ebay item. If you really want to buy one a friend has one in his ebay yet to be listed pile in his basement. Where you are right now starts with a report about the primary voltage on the coil and what you are going to do to fix the missing wire from the starter.

Second thing I do not understand is how you are reporting no spark. Are you attaching a timing light to the secondary coil wire and seeing no blinking lights from the timing light? You have confirmed the timing light is working properly on another car?

What is the point of a resistor wire? This cirsuit design is as old as points and a condensor. In days of old GM had a resistor mounted on the firewall. VW chose to get fancy with the resistor wire. It makes the points last longer. If you make that wire straight battery voltage all the time you will go through points much faster.

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Note that the coil for a car with electronic ignition is not the same as a coil for a car with points. If try to troubleshoot by switching out incompatible parts you might blow up parts on either the donor or donee car.

Terminal 15 is what I am refering to as primary voltage. Terminal 1 on the coil is attached to the distributor. You should measure the resistance between 15 and 1. then measure the resistance between 1 and the high tension also refered to as 4 but probably no marked. Note that the values you report may not be helpful if your meter is not accurate.

All of this troubleshooting and how things work is covered in a Bentley. Just do an ebay search for bentley rabbit.

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Reply to
Jim Behning

A cheapie analog meter may be best here. I would measure the voltage at the low voltage terminal on the coil while cranking. The voltage should go up and down both when the meter is referenced to the positive terminal of the battery or ground. The dwell could be estimated from the percent of time that the meter is one way vs the other as you crank. The results should be informative. I would use the 25 volt range.

Hold off in case somebody posts that is likely to damage the meter due to inductive kickback.

Reply to
Tom's VR6

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Thanks for all of that info!! Before I do any metering (which I have no experience with), I was considering replacing the coil with the CORRECT coil for this car. But I've heard that they rarely go bad. Thoughts? Also, I will probably switch the starters back so I can attatch that "extra" wire to the starter -- just like it used to be. Although, if the stray wire is the problem, I wonder why it worked as long as it did? Perhaps I shorted something? Do the two wires that come off the starter go directly to the ignition coil? Also, I've been testing for spark on the high-voltage wire that goes from the coil to the distributor cap (the one that looks like a spark plug wire). This is where I'm seeing intermittent "spark". I used this same method on my working rabbit and the gun flashed constantly/consistently, so I assume this is an acceptable method.

Reply to
Fred Mann

Well battery voltage is a factor. That wire might be more usefull when you get cold batteries that do not have a lot of voltage.

If you do not know how to use a voltmeter then you are in for a horrid adventure. Meter useage is pretyy simple for basic test. Black to ground. Red to positve. Read voltage and report back. Don't try to use meter to read secondary voltage.

To read resistance you set the meter to resistance and connect to terminals mentioned. Report back.

You can google how to use a multimeter for some 101 lessons.

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Reply to
Jim Behning

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